1. Field of the Invention
The present invention relates to a method and apparatus for increasing the filling capacity of tobacco by infrared radiation heating of water expanded tobacco under freeze-drying conditions.
2. Description of the Prior Art
The tobacco art has long recognized the need to effect the greatest possible degree of expansion of tobacco while maintaining desirable handling and smoking characteristics. Correspondingly, numerous attempts have been made in the art to effect such expansion of tobacco, frequently by treatment of the tobacco with an agent which expands greatly during evaporation or after a decrease in pressure.
One attempt in the prior art to expand tobacco is disclosed in U.S. Pat. No. 1,789,435 which teaches a method for expanding the volume of tobacco in order to make up the loss of volume caused in curing the tobacco leaf. The tobacco is contacted with a gas such as air, carbon dioxide, or steam under pressure and, upon release of the pressure, the tobacco tends to expand limitedly between 5% and 15% by volume.
Prior art disclosures are also available which teach that tobacco may be expanded by addition of water to the tobacco which causes the tobacco to swell following which the contained moisture is evaporated to set the expansion.
Another attempt to expand tobacco has been by use of carbohydrates as a means to improve puffing of tobacco stems. In this process the tobacco stems are soaked in an aqueous solution of carbohydrate following which they are heated to set the tobacco expansion.
Volatile organic liquids have also been disclosed in the prior art as means to effect expansion of tobacco.
Methods have also been proposed in the prior art to effect tobacco expansion by use of ammonia and carbon dioxide gases. Carbon dioxide has also been used in the liquid state as a means of expanding tobacco as well as other organic plant substances. Typically, such processes require immersing, for example, tobacco in a pool of liquid carbon dioxide wherein the tobacco is steeped in the liquid carbon dioxide following which the tobacco is heated, preferably using super-heated steam to set the expansion. This method, however, invites various disadvantages by requiring needlessly large quantities of liquid carbon dioxide relative to the amount of carbon dioxide which actually impregnates the tobacco. Furthermore, components of the tobacco such as sugars and flavoring materials are typically extracted by the use of excess liquid carbon dioxide and a corresponding loss of tobacco flavor is experienced.
Tobacco stems and veins have been expanded heretofore by treatment with steam such that moisture penetrates the fibrous structure and by then heating the moisturized stems, or exposing them to a vacuum, the fibrous mass becomes expanded. The same treatment has been tried with laminar tobacco, but when the treated product is dried the expansion decreases and the dried tobacco has substantially the same volume which existed before treatment.
A more effective expansion of laminar tobacco such as shreds used for cigarettes has been achieved heretofore by soaking the shreds in a relatively large volume of cold water and thereafter freeze-drying the water expanded tobacco. The amount of cold water required has been about eight times the weight of the tobacco treated, and soaking in this amount of water for close to one hour, or even more, has been necessary to obtain the desired expansion of the tobacco. During this soaking period a considerable amount of the water-soluble constituents of the tobacco are extracted in the water. Stirring of the tobacco during the soaking period has been attempted and found to be undesirable because of the tendency of the tobacco shreds to tangle or mat. When expansion of the tobacco is effected in this way, the entire mass of water and tobacco is frozen into a single block of ice in which the tobacco is concentrated at the top of the ice block and the water-soluble extractives are concentrated in the lower main body of the ice block. The ice block is next evaporated in a vacuum with resulting formation of a dry product consisting essentially of the the expanded shreds lying on a film-like layer of water-extracted solids which must then be re-combined. The resulting product is characterized by an objectionable amount of tackiness because of the hygroscopicity of the film-like component which forms a surface layer on the shred portion of the re-combined material.
The rate of heat transfer in a vacuum using conventional freeze-drying methods which employ heated shelves, plates or platens is relatively slow because metal-to-metal contact is usually only at three points and tobacco strips or shreds have surface irregularities which allow only minimal surface contact with a metal tray. It is well known that heat transfer by conduction is restricted to the contact points and that heat transfer by convection is extremely limited at reduced pressures. In conventional freeze-drying, the shelves are usually overheated to improve the drying rate, but this overheating can readily cause contact point scorching.
It has now been found that by practice of the present invention evaporative freeze-drying of tobacco on infrared transparent trays under vacuum may be more effectively achieved by means of infrared radiation heating. It is found that infrared radiation transfers heat exceptionally well in a vacuum and will present the heat to the entire exposed surface of the tobacco rather than to the usual three metal-to-metal contact points. Also, scorching will be avoided by direct infrared radiation from a controlled source. In addition, the infrared radiation increases the heat transfer rate and there is a corresponding decrease in the freeze-drying time. Further advantages of infrared over other forms of radiation such as microwave or dielectric heating include the absence of corona discharge, and the simplicity of producing, directing and using infrared energy, particularly in a vacuum system.
It has also now been found that by practice of the present invention tobacco expansion may be effected which yields a non-tacky puffed product admirably suited for further processing into a smoking product.